1. Field of the Invention
The present invention relates to automatic gain control circuits and more particularly to such circuits which vary the impedance of a device for shunting current from the input of an amplifier.
2. DESCRIPTION OF THE RELATED ART
Transimpedance amplifiers produce an output voltage signal proportional to an input current signal. Such amplifiers are typically implemented by providing a feedback resistor across the input and output of a voltage amplifier.
A typical use for transimpedance amplifiers is in optical transmission systems. An optical source, such as a laser or light emitting diode, is used to transmit information by modulating the intensity of the light source. An electrooptic device such as a PIN diode or avalanche photodiode receives the optical signal and converts the same to a current which is applied to the input of the transimpedance amplifier. The amplifier thus produces a voltage proportional to the diode current. In optical transmission systems the information transmitted is usually digital and generally is in the form of a pulse train.
Optical transmission systems are typically designed to provide an electrical output signal which has a predetermined amplitude. In the prior art, an automatic gain control (AGC) is used in conjunction with the transimpedance amplifier described above in order to produce an output voltage which comprises a pulse train switched in accordance with the optical source and having a substantially constant amplitude when the input signal is larger than a preselected threshold.
One such AGC circuit is shown in FIG. 37 of Williams U.S. Pat. No. 4,540,952 for a nonintegrating receiver. Such an AGC circuit effectively varies the gain of the transimpedance amplifier in order to maintain the output signal at the desired amplitude for signals above the threshold.
In Williams, a FET is used as a resistive shunt to reduce the value of the current applied to the input terminal of the amplifier in order to increase dynamic range. One side of the FET is connected to the amplifier input, the other side is connected to a bias voltage and the gate is connected to the output of the AGC drive circuit. The Williams device is illustrated schematically in FIG. 1 of the accompanying drawings. In Williams, the bias voltage varies responsive to the average amplifier output voltage. Other prior art circuits use a fixed bias voltage.
The AGC drive circuit produces a control signal which varies in response to the amplifier output. For high amplifier input signal levels, more amplifier input current is shunted thereby increasing the dynamic range of the amplifier. In order to be effective, such a FET must typically have a resistance of 10 ohms or less. Such a FET is typically one to several thousand microns wide. This takes up a tremendous amount of space on a chip and also adds a large amount of unwanted capacitance which lowers the bandwidth and degrades the noise performance.